Acoustic Guitar Part 1

Building an acoustic guitar is a very satisfying project that is within reach of most people with a modicum of woodworking experience. I have speed-built a guitar within a week, but for a more considered approach it is more usual to take three to four weeks. Most tools required are in the general woodworking shop; some simple ones that are luthiery-specific can be easily made or adapted from other tools. Access to a thickness sander is almost essential, but with patience there's not much that can't be achieved by planing/ scraping/sanding. Photographs for this article are from student guitars built during the acoustic guitar building courses at the Centre for Fine Woodworking, and also from previous guitar builds I have completed.

Tonewood selection

Tonewoods are named for their ability to contribute their own tonal character to the sound that your guitar produces and should be selected to suit the overall sound that you are looking to produce from your guitar. Traditionally the back and sides are made from the same timber (usually a dense hardwood). Commonly used exotic woods for back and sides are East Indian rosewood (bias towards lush dark tones), mahogany (brighter than rosewood, quite punchy sounding), maple (can sound a bit thin and trebly) and walnut (somewhere between mahogany and rosewood). Native NZ timbers could also be used if you can find a wide enough quartersawn plank. For example matai, totara or even rimu can make a fine sounding guitar. The guitar soundboard or top is from resonant softwood such as spruce or cedar. There are several species of spruce, all with slightly differing qualities and prices. Sitka or Engelmann spruce are the most commonly available species of spruce from luthiery supply companies, and these will produce a top-quality instrument. Adirondack or Red spruce was used on highly sought-after prewar Martin guitars, and is considered by some to be the best spruce for guitar tops, but it is getting rare (and therefore expensive, and somewhat variable in quality). The indications about how these woods sound is a bit of a generalisation, and the way that the guitar is built (size, bracing, wood thicknesses) and the skill of the luthier in using the qualities of these woods also play a major part in the final sound. Other important points to make about the tonewoods is that ideally they should be perfectly quarter-sawn (grain lines at right angles to the surface), air-dried, and bookmatched. For a first guitar I would recommend mahogany for the back/sides and sitka spruce for the top.

Guitar design

The guitar we build on the Centre For Fine Woodworking course is a smallbodied guitar ideal for fingerstyle playing or strumming. The body of the guitar is built up in a mould made of layers of 18 mm MDF which are accurately copy-routed to a master template defining a half outline of the guitar, then screwed together on a baseboard, working to a clearly marked centreline. Use parcel tape to cover the area of the baseboard that will be under the heel and tail blocks, and also run a strip up the inside wall of the mould on the centreline to avoid glue squeeze-out sticking the sides to the mould. Prior to building, also make an accurate template out of 4 mm ply that defines the outline of the body shape and on it mark all important dimensions from your plan: centreline, centre of soundhole, soundhole dimensions, rosette dimensions, bracing, and also where the bridge plate, saddle and bridge pins will be. Throughout this article, the words “strut” and "brace” for the support strips of timber are almost interchangeable, although it is conventional to consider the back to have struts and the top braces.

Preparing the sides

Blanks for the sides are normally cut to approximately 820 mm long by 110 mm wide, and at a thickness of 3-4 mm. They are bookmatched pieces of wood. It is important to mark them in some way to ensure that the bookmatched sides are on the outside in order to get a visually pleasing match up at the bottom of the guitar. The side to which the soundboard will be glued is planed flat. The edge that is going to be glued to the back is not straight, as the guitar body is thicker at the bottom (tail) than at the heel (where the neck joins the body). This lengthwise taper is not even; there is a high point at the waist, which helps spring the back into a lengthwise arch. Also, when the back has struts glued on, these also have an arch in them. The crosswise and lengthwise arches combine to give the back a slight domed shape, and the guitar is shallower at the heel end than at the tail end. Because of this the sides have to be shaped to accommodate the dome of the back, resulting in the sides being trimmed lower at the outer point of the curves of the upper and lower bout of the guitar, and being tapered lengthwise. Taking measurements from plans or from an existing guitar of similar body style will serve as a good approximation that can be refined later when fitting the back. When taking measurements, remember to take into account the top and back thickness to get the width for sides or you will end up with a guitar that is approximately 5 mm thicker overall than planned. Once you are happy with the measurements, draw onto the side blanks and bandsaw off the excess. Sides are thicknessed to 2 mm; a thickness sander is ideal. Running wood to this thickness through a thickness planer is not really viable, there is too much danger of the spinning blades flexing the wood to the point that it breaks.

Bending the sides

Make a half guitar template out of 18 mm ply that is 2 mm smaller all around than the outline of your guitar. Bending the sides involves developing a feel for how the timber you are using will respond to heat and moisture. Some practice using narrower offcuts is a worthwhile investment of time and effort. We use an aluminium heating block supplied by one of the luthiery supply companies. It is possible to get by using a blowtorch and some steel pipe clamped in a vice but aside from obvious safety issues, it is difficult to regulate the temperature so that the wood doesn't get scorched (this will need heavy scraping/sanding to remove, bearing in mind that you only have 2 mm of wood to work with). I normally start bending the sides at the waist of the guitar. Mark with pencil on the outside face of the sides where the waist of the guitar will be, moisten the wood with a spray bottle, and place the side on the hot bending iron, move the side to and fro on the hot iron to distribute the heat and moisture. Apply light pressure with a hand either side of the hot iron, and increase the pressure slightly as the top surface of the wood starts to steam and dry out. At this point the wood starts to feel more flexible or a bit plastic. Don't try and force things to get the curve for the waist done in one fell swoop. It usually takes three to five applications of water and reapplication of heat to get the curve right. Some moisture is essential to the bending process, too dry and the wood becomes more brittle, too much and the heating iron cools down and the wood doesn't reach the point where the lignin fibres can slide over each other. Once the waist curve fits the template, you can flip the side over and start moving around the curve of the upper half, then the lower half. It is tempting to try and rush things, but accept that it will take some time initially (several hours). It is important to get an accurate fit to your template as this will define the outline shape of your guitar.

Fitting sides to mould

The guitar will be built face down, so place one bent side into the mould with the straight soundboard edge facing downwards. The side should be a fairly snug fit to the sides of the mould. Any major discrepancies might need a bit of tweaking on the bending iron to ensure a good fit. Clamp this side in the mould and transfer the centreline (at both ends) from the baseboard to the inner surface of the side. Remove the side, square the line across and trim to the line with a razor saw. Repeat for the other side. Both sides should now be a snug fit in the mould.

Making and fitting heel and tail block

The Heel block (at the neck end of the body) dimensions are 70 mm wide by 50 mm long, and depth to suit the guitar being built. It is best to make them overdeep by 5 mm or so in order that the edge that the back is glued to can be planed to suit the arch of the back. I usually laminate two longer pieces of wood together to give an overall thickness of 70 mm, then square up and use the tablesaw to cut off 50 mm lengths to make several blocks at a time. The heel block is drilled (6.5 mm dia) on a drill press to maintain square with two holes to receive the neck bolt. I also remove some material on the bandsaw to create a rough mortise that will later be trimmed by hand to receive the tenon on the neck. The heel block is glued onto the sides such that the end grain of the heel block is the glue surface. This is not a particularly strong way of joining wood, but there are also two bolts running through it into the neck and the top and bottom surfaces are also glued so there is plenty of strength. Prior to gluing, the glue surface of the heel block is planed to give it a curve to match the outline shape of the guitar. Dry-clamp the heel block into position and check that the centreline of the block matches up with the centreline of the baseboard, the block sits down flat on the baseboard, and the curve planed into the glue surface is accurate and a good fit to the sides/ mould. If all is OK, remove clamps, apply glue (Titebond or similar) and clamp back in position. Clean up excess glue with a damp cloth. Using the two predrilled holes in the heel block as a guide, drill through the sides from the inside. The tailblock can be made of plywood, or a 3 ply laminate of spruce or similar, normally about 60 mm wide, and 18 mm thick, and about 2 mm greater than the depth of the guitar being built. This is shaped in a similar way to the heel block to match the outline shape at the tail end and then glued in, ensuring it sits down flat and tight against the baseboard of the mould.

Linings

Linings are glued on to the sides to add stiffness and to give a wider gluing area to attach the back and soundboard. The linings are thin strips kerfed (cut nearly through) at regular intervals. Kerfing the linings makes them flexible so they can be glued on without steam bending. Many wood species can be used, spruce, walnut, mahogany, Tasmanian blackwood. Lining is dry-fitted and trimmed to length to fit between the heel and tail blocks for both halves of the guitar; glue is brushed onto the glue side of the lining and then attached using clothes pegs or small spring clamps. It is important to glue the lining on so that it stands proud of the side by approximately 2 mm, so that it can be trimmed back later to accommodate the back doming. Glue squeeze-out below the lining is dealt with now.

Back and top glue up

Normally the back and top are made from bookmatched pieces of wood, so that the grain pattern runs symmetrically about the centreline. The two halves are edgeglued before bringing the pieces down to their final thickness (for the back approximately 2.5 mm, and for the top somewhere between 2.5 and 3.0 mm). Before edge-gluing, the centre joint is 'shot' with a long plane to give a smooth and true edge that can be glued to give an invisible join. For gluing, pressure has to be applied inwards and this can be done using wedges that are tapped in against a rail at either side or by using small nails that are placed just to the outside of the back or top to be joined, and after placing weights either side of the centre join to prevent lifting as pressure is applied, tapping them inwards to push the two halves together closely. Remember to place greaseproof paper or something similar under the centre join to prevent gluing it to the baseboard.

Thicknessing the back

Once the glue has dried, lift off the baseboard, and use a cabinet scraper to remove the larger blobs of dried glue, as it will clog the thickness sander. Run through the thickness sander until the outside face is sanded evenly all the way across. Turn the back over and thickness to somewhere between 2.2 and 2.5 mm. Using your guitar outline template, mark the outline of the guitar and bandsaw the waste off about 5 mm outside this line. The inside surface of the back is scraped and sanded until smooth and the centre join is reinforced with a strip of spruce. This can be made up of 10 mm strips approx 4 mm thick cut cross-grain from the excess soundboard material. It is trimmed to length later so it is made overlong at this stage. At the CFW we use a vacuum bagging system for gluing on most of the reinforcing and bracing of the back and the soundboard. It allows strong and even clamping pressure without distorting the thin wood. To glue on the strip apply glue to one side of the reinforcing strip and use masking tape to position it centrally over the glue join of the back, insert in the vacuum bag and turn on the vacuum pump, check that nothing has moved and leave to dry. Glue squeeze-out is cleaned up and the centre strip is planed down to approx 3 mm height and shaped so that the top is rounded crossways (using a chisel, small plane and sandpaper). The back with centre strip is placed on the guitar (be careful to align it with the centreline of the body) and a pencil mark is made at each end of the centre strip at the heel and tail end of the guitar. From these marks transfer the measurements for the inside surfaces of the tail and heel blocks to get the internal length of the guitar. The struts are spaced equidistant in this length, and their positions are marked out on the centre strip. A razor saw is used to cut through the reinforcing strip either side of the strut, and chisel out the waste in order to create a notch in which the strut can sit. Back struts (spruce, mahogany, walnut) are cut from quarter-sawn stock so that the grain stands vertically in the strut for maximum stiffness. Dimensions are roughly 7 mm wide, by 16 mm high and length to suit the width of the guitar. There are four struts and they are spaced equidistant in the gap between the tailblock and the end block. The struts have an arched glue surface to the back so that they induce a crossways arch in the back when they are glued on. To do this a template is made from 3-4 mm thick ply to the length of the longest strut, the centre point is marked and a curve is drawn on the template to give a 4 mm arch (i.e. the line ends up 4 mm away from the baseline at each end of the strut), and cut to this line using a block plane. This curve is marked on the side of all four struts using a fine pencil, and the strut is planed to the line, ensuring that the glue surface is kept square to the sides of the strut. It is worth writing 'glue' on the curved surface to ensure that the strut is used the right way up. Choose a low humidity day to glue on the back struts. The struts can now be glued on in the vacuum bag ensuring that they are square to the centreline. Once dry, the ends of the struts are scalloped (chiselled down to approx 3-4 mm in height), and the top of the struts shaped to give a rounded profile.

Gluing the back

Place the back onto the sides in the mould. At this stage it will be obvious that the tailblock and heelblock are too high, and also that the scalloped ends of the struts prevent the back sitting down on the linings. Using a sharp block plane, plane down the heel block and the tail block until they are a similar height to the linings, and so that they help support a lengthwise arch in the back. Realign the back on the sides and mark on the side of the guitar where the sides of each strut fall. Using a coping saw cut a notch 4 mm deep through the lining and the side to house each strut end. It is important to go no deeper than 4 mm as the bindings on the guitar edge will be 7 mm deep that will cover 4 mm plus back thickness (2.5 mm). Any deeper and the ends of the struts will not be covered. Refit the back and check that the strut notches are in the correct place and that they allow the back to sit perfectly on the centreline. The centre strip on the back will need trimming to length so that it will sit down between the heel and tailblocks. Also at this stage the linings will still be slightly high, as will the tail and heel blocks. Use a block plane to trim the top surface of the linings and the blocks to suit the dome of the back and to enable it to sit tight down on the sides. The strut ends or notches may also need tweaking to get the right fit. When the fit is good and the dome of the back is maintained it is ready to glue on. Apply glue to to the surface of the lining, and to the top surface of the heel and tailblock. The back is lowered into place and clamped at the heel and tailblock first (using large 6 mm thick clamping cauls to spread the pressure over the block). Check centrelines, and then clamp around the body, using 30 mm square clamping cauls of thin ply - it is important to position the cauls and clamps so that the clamping pressure comes down onto the lining and the side, rather than too far inboard which will depress the back, or too far outboard which will induce too much arch. Do not overtighten the clamps, they are squeezing down onto the edge of a piece of wood which is only 2 mm thick. When the glue is dry, remove clamps and ease the body out of the mould. Glue kerfed linings onto the soundboard edge of the sides and once dry, plane these smooth and flush to the top, with a slight bevel upwards to accommodate a slight dome in the soundboard.

Soundboard

After joining, the outside face of the soundboard is smoothed using the thickness sander, taking care to remove only the minimum of material. Draw on the outline of the guitar using the template, and aligning on the centreline. Measure from the template where the centre of the soundhole will be and mark this on the top. A circle cutter with a small spike in the underside will allow accurate cutting of both the inside and outside diameter of the rosette channel to a depth of 1-1.5 mm. The channel width is cut so that the material used for the rosette, for example a strip of herringbone-pattern purfling, is a snug fit otherwise glue lines will show around the edge of the rosette. The waste between the cuts is chiselled out using a narrow chisel and a small router plane. The herringbone strip can be bent to suit using the side-bending iron. Once a good fit has been achieved, glue is applied to the channel and the rosette pushed down into this. The top surface of the rosette should be slightly proud of the soundboard, and once the glue has dried it is scraped/ sanded flush with the soundboard. The soundboard is trimmed on the bandsaw to 5 mm outside the outline of the guitar then it is thicknessed from the reverse side to the rosette. Top thicknessing is critical to the finished sound of the instrument, and will vary depending on wood used, and how stiff that particular piece of wood is. Sitka spruce is normally thicknessed down to somewhere between 2.6 and 3.0 mm. Once thicknessed, the soundhole can be cut out from the rosette side using a circle cutter. Leave a margin of about 5 mm between the inner edge of the rosette and the soundhole. The soundboard is now too thin to take the tension of the strings without collapsing, so has to be selectively reinforced with braces. The way these are glued on and shaped will influence not only the structural stability of the guitar soundboard but also the sound. Mark the layout of the bracing on the underside of the soundboard. All the bracing is sawn from spruce or similar so that is quarter-sawn (grain runs vertical to the soundboard for maximum stiffness). The two large braces that make up the X brace will take the bulk of the load and are 18 mm high by 8 mm wide, the two lower braces are called harmonic bars and are smaller (12 mm high, 6 mm wide), the two short braces either side to the outside of the X brace are called finger bars (6 mm x 6 mm). There is also a bridge plate that sits between the X braces and the harmonic bars which is made from 3 mm thick hardwood (maple, rosewood, walnut) and an additional large brace (14 mm high x 12 mm wide) that runs crosswise above the soundhole with two small braces supporting the sides of the soundhole (6 mm x 6 mm). Similar to the back braces which had an arch of 4 mm planed into them, the X braces and the harmonic bars for the top have an arch of 3 mm planed on one surface in order to induce some doming to the soundboard. The larger cross-brace above the soundhole is arched by only 2 mm and it has a U-shaped hole cut into it on the centreline through which an Allen key can be inserted to adjust the truss rod. Cut a half lap joint in each X brace where they cross over, so that they can be notched into each other and so that the arched surface will be the gluing surface to the soundboard. It is best to glue on the struts on a low humidity day. It is easiest to glue the braces on using the vacuum bag in stages, normally the X brace assembly first, along with the bridge plate, then the two harmonic bars. Once these are glued on, the smaller braces can be clamped using small woodworking clamps, as they can be reached through the soundhole or from the edge of the soundboard. The braces are now shaped using chisels, spokeshaves, small planes and sandpaper to remove some of their weight while maintaining stiffness. The top cross brace is typically rounded, and the end scalloped down to 3 mm to notch into the linings, the X braces are slightly rounded around the central join, but scalloped down to 3 mm at their ends either side of the soundhole. Below the soundhole there are numerous options, from running a long taper out to the ends which are left 3 mm high, or removing more material to produce a double scallop. There is a compromise between strength and soundboard flexibility and it generally better to not be too aggressive in removing material until some experience has been gained. None of the other braces are notched into the linings and these can be tapered down to nothing about 10mm short of where the lining starts. It is interesting to tap the bridgeplate and hear the difference in sound as the braces are shaped.

Gluing sound board

Place the soundboard onto the guitar body, line it up on the centreline, and ensure that is placed accurately longitudinally (measure from the soundhole edge), mark where the X brace ends fall on the sides, and also the ends of the large top cross strut, and remove notches from the body with a coping saw to house these. Once the top sits down onto the linings all the way around, glue the top on in a similar way to the back, using plywood pads as clamping cauls on the top. Once dry, the clamps are removed and the overhang of the top and back are removed using a spokeshave or block plane. It is also possible to use a router with a flush cutting bit. • In the next issue we add the neck and finish the guitar.